Bug 1834718: Don't process a duplicate DocAccessible::ContentRemoved call for...

Bug 1834718: Don't process a duplicate DocAccessible::ContentRemoved call for a DOM node we've already processed. r=morgan, a=dmeehan

ContentRemoved recursively walks both AllChildrenIterator and direct DOM children.
In addition, we might get duplicate notifications from DOM and layout, plus PruneOrInsertSubtree might do a recursive walk and it too calls ContentRemoved.
To avoid this duplicate processing, keep a set of removed DOM nodes on the DocAccessible which we clear after mutation events are processed.

Differential Revision: https://phabricator.services.mozilla.com/D196707